Using the specific gravity determinations on abnormal uptake of water by the brain tissue was followed in gerbils which were subjected to cerebral ischemia by occlusion of the common carotid artery at the neck. The significant edematous change was demonstrable in various brain structures subjected to ischemia within 10 minutes following occlusion. In the postischemic periods, animals which developed necrotic changes revealed extremely low specific gravity of the affected tissue and a conspicuous damage of the blood-brain barrier, as demonstrated by the extravasation of the Evan's blue tracer.